Phase change memory cells having vertical channel access transistor and memory plane

ABSTRACT

Memory devices are described along with methods for manufacturing. A memory device as described herein comprises a plurality of word lines overlying a plurality of bit lines, and a plurality of field effect transistors. Field effect transistors in the plurality of field effect transistors comprises a first terminal electrically coupled to a corresponding bit line in the plurality of bit lines, a second terminal overlying the first terminal, and a channel region separating the first and second terminals and adjacent a corresponding word line in the plurality of word lines. The corresponding word line acts as the gate of the field effect transistor. A dielectric separates the corresponding word line from the channel region. A memory plane comprises programmable resistance memory material electrically coupled to respective second terminals of the field effect transistors, and conductive material on the programmable resistance memory material and coupled to a common voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high density memory devices based on phase change memory materials, including chalcogenide based materials and on other programmable resistance materials, and methods for manufacturing such devices.

2. Description of Related Art

Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistance material to form nonvolatile memory circuits, which can be read and written with random access.

The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the molten phase change material and allowing at least a portion of the phase change material to stabilize in the amorphous state.

The magnitude of the current needed for reset can be reduced by reducing the size of the phase change material element in the cell and/or the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material.

One approach to reducing the size of the phase change element in a memory cell is to form small phase change elements by etching a layer of phase change material. However, reducing the size of the phase change element by etching can result in damage to the phase change material due to non-uniform reactivity with the etchants which can cause the formation of voids, compositional and bonding variations, and the formation of nonvolatile by-products. This damage can result in variations in shape and uniformity of the phase change elements across an array of memory cells, resulting in electrical and mechanical performance issues for the cell.

Additionally, it is desirable to reduce the cross-sectional area or footprint of individual memory cells in an array of memory cells in order to achieve higher density memory devices. However, traditional field effect transistor access devices are horizontal structures having a horizontally oriented gate overlying a horizontally oriented channel region, resulting in the field effect transistors having a relatively large cross-sectional area which limits the density of the array. Attempts at reducing the cross-sectional area of horizontally oriented field effect transistors can result in issues in obtaining the current needed to induce phase change because of the relatively low current drive of field effect transistors. Although bipolar junction transistors and diodes can provide a larger current drive than field effect transistors, it can be difficult to control the current in the memory cell using a bipolar junction transistor or a diode adequately enough to allow for multi-bit operation.

It is therefore desirable to provide memory cells having field effect transistor access devices for use in high-density memory devices and providing the current necessary to induce phase change, as well as addressing the etching damage problems described above.

SUMMARY OF THE INVENTION

A memory device as described herein includes a plurality of bit lines and a plurality of word lines overlying the plurality of bit lines. The device includes a plurality of field effect transistors. Field effect transistors in the plurality of field effect transistors are arranged vertically, comprising a first terminal acting electrically coupled to a corresponding bit line the plurality of bit lines, a second terminal overlying the first terminal, and a channel region separating the first and second terminals. A corresponding word line acting as a gate of the field effect transistor is adjacent to, or surrounds, the channel region. A dielectric separates the corresponding word line from the channel region. The device further includes a memory plane comprising programmable resistance memory material electrically coupled to respective second terminals of the field effect transistors, and conductive material on the programmable resistance memory material and coupled to a common voltage.

In embodiments the vertical field effect transistors can be formed within a via in the corresponding word line such that the allocated cross-sectional area of memory cells in an array can be determined entirely by dimensions of the word lines and bit lines, allowing for a high memory density of the array.

Additionally, the channel region and the first and second terminals are arranged vertically so that the field effect transistor can have a small cross-sectional area while also providing sufficient current to induce phase change. The length of the channel of the device is determined by the height of the channel region and can made small, while the width of the channel of the device is dependent upon the circumference of the channel region and can be made relatively large compared to the length. Thus, a relatively large width-to-length ratio can be achieved such that higher reset current can be obtained.

Additionally, in embodiments the programmable resistance memory material can be a blanket layer of programmable resistance memory material contacting the plurality of electrodes of memory cells of the array such that the array is not subject to the etching damage problems discussed above.

Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description, and the claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a potion of a memory cell array implemented using memory cells having field effect transistors with vertical channels and memory elements comprising programmable resistance material of a memory plane.

FIGS. 2A-2B illustrate cross-sectional views of a portion of an embodiment of memory cells arranged in the array of FIG. 1.

FIGS. 2C and 2D illustrate cross-sectional views an alternative embodiment in which the electrodes of the array of FIGS. 2A and 2B are omitted and the memory material of the memory element extends within the opening in the dielectric to contact the conductive cap.

FIGS. 3A-11B illustrate steps in a fabrication sequence for manufacturing the array of memory cells of FIGS. 2A-2B.

FIGS. 12-18B illustrate an alternative manufacturing embodiment to that of FIGS. 5-9.

FIGS. 19A-19B illustrate cross-sectional views of a portion of an embodiment of memory cells arranged in the array of FIG. 1.

FIGS. 20-28B illustrate steps in a fabrication sequence for manufacturing the array of memory cells of FIGS. 19A-19B.

FIG. 29 is a simplified block diagram of an integrated circuit including a memory array implemented using memory cells having a memory plane overlying vertical channel field effect transistor access devices.

DETAILED DESCRIPTION

The following description of the disclosure will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the disclosure to the specifically disclosed embodiments and methods, but that the disclosure may be practiced using other features, elements, methods and embodiments. Preferred embodiments are described to illustrate the present disclosure, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. Like elements in various embodiments are commonly referred to with like reference numerals.

FIG. 1 illustrates a schematic diagram of a portion of a memory cell array 100 implemented using memory cells having field effect transistors with vertical channels and memory elements comprising programmable resistance material of a memory plane as described herein.

As shown in the schematic diagram of FIG. 1, each of the memory cells of array 100 includes a field effect transistor access device and a memory element arranged in electrical series, the memory elements capable of being set to one of a plurality of resistive states and thus capable of storing one or more bits of data.

The array 100 comprises a plurality of bit lines 120 including bit lines 120 a, 120 b, 120 c, 120 d extending in parallel in a first direction and in electrical communication with bit line decoder 160. The field effect transistors of the array 100 have first terminals acting as a source or drain coupled to a corresponding bit line 120.

A plurality of word lines 130 including word lines 130 a, 130 b, 130 c, 130 d extend in parallel in a second direction and are in electrical communication with word line decoder/driver 150. As described in more detail below with respect to FIGS. 2A-2B, the word lines 130 overly the bit lines 120. The word lines 130 are adjacent to the vertical channels of the field effect transistors to act as the gate terminals of the transistors. In alternative embodiments, the word lines 130 may completely or partially surround the channels, or otherwise lie adjacent the channels, and are separated from the channels by a gate dielectric layer.

The memory elements of the memory cells of array 100 comprise respective portions of the programmable resistance memory material of a memory plane (described in more detail below with respect to FIGS. 2A-2B) overlying the bit lines 130 and word lines 120 of the array 100. The memory elements of the memory cells are electrically coupled to the second terminals of the field effect transistors by electrodes 250 that provide a small contact area between the field effect transistors and the memory elements.

The memory plane includes conductive material 140 (described in more detail below with respect to FIGS. 2A-2B) on the programmable resistance memory material. The conductive material 140 of the memory plane is electrically coupled to a memory plane termination circuit 170. In the illustrated embodiment the memory plane termination circuit 170 is a ground terminal, but may alternatively include a voltage source for applying a common voltage other than ground to the conductive material of the memory plane.

Memory cell 110 is representative of memory cells of array 100 and comprises field effect transistor 115 and phase change memory element 125 arranged electrically in series between the memory plane and the corresponding bit lines 120. The word line 130 b acts as the gate terminal of the transistor 115, and the first terminal (acting as the source or drain of the transistor 115) is coupled to bit line 120 b. The memory element 125, comprising programmable resistance memory material of the memory plane overlying the word lines 130 and bit lines 120, is electrically coupled between the second terminal of the transistor 125 and the conductive material 140 of the memory plane.

Reading or writing to memory cell 110 of array 100 can be achieved by applying an appropriate voltage to the corresponding word line 130 b and an appropriate voltage or current the corresponding bit line 120 b to induce a current through the memory element 125. The level and duration of the voltages/currents applied is dependent upon the operation performed, e.g. a reading operation or a writing operation.

In a reset (erase) operation of the memory cell 110, a reset pulse applied to the word line 130 b and the bit line 120 b induces a current through the memory element 125 to cause a transition of an active region of the memory element 125 into an amorphous phase, thereby setting the phase change material to a resistance within a resistance value range associated with the reset state. The reset pulse is a relatively high energy pulse, sufficient to raise the temperature of at least the active region of the memory element 125 above the transition (crystallization) temperature of the phase change material and also above the melting temperature to place at least the active region in a liquid state. The reset pulse is then quickly terminated, resulting in a relatively quick quenching time as the active region quickly cools to below the transition temperature so that the active region stabilizes to a generally amorphous phase.

In a set (or program) operation of memory cell 110, a program pulse is applied to the word line 130 b and the bit line 120 b of suitable amplitude and duration to induce a current through the memory element 125 sufficient to raise the temperature of at least a portion of the active region of the memory element 125 above the transition temperature and cause a transition of at least a portion of the active region from the amorphous phase into a crystalline phase, this transition lowering the resistance of the memory element 125 and setting the memory cell 110 to the desired state.

In a read (or sense) operation of the data value stored in the memory cell 110, a read pulse applied to the corresponding word line 130 b and the corresponding bit line 120 b of suitable amplitude and duration to induce current to flow through the memory element 125 that does not result in the memory element 125 undergoing a change in resistive state. The current through the memory cell 110 is dependent upon the resistance of the memory element 125 and thus the data value stored in the memory cell 110. The data valued stored in the memory cell 110 may be determined, for example, by comparison of the current on bit line 120 b with a suitable reference current by sense amplifiers of block 165. Alternatively, the data value stored in the memory cell 110 may be determined, for example, using source side sensing by comparison of the current on the conductive material 140 of the memory plane with a suitable reference current.

FIGS. 2A and 2B illustrate cross-sectional views of a portion of an embodiment of memory cells (including representative memory cell 110) arranged in the array 100, FIG. 2A taken along the word lines 130 and FIG. 2B taken along the bit lines 120.

The array 100 includes a single-crystalline substrate 200 comprising a well 205 having a first conductivity type and bit lines 120 within the well 205. The bit lines 120 extend in a first direction into out of the cross-section illustrated in FIG. 2A and are separated by dielectric trench isolation structures 232 within the well 205. The bit lines 120 comprise doped substrate material having a second conductivity type opposite that of the first conductivity type. In the illustrated embodiment the doped substrate material of the bit lines 120 comprises high doped N-type (N+) material of the substrate 200, and the well 205 comprise doped P-type material of the substrate 200.

The field effect transistor 115 of the memory cell 110 includes a first terminal 122 comprising doped semiconductor material on the corresponding bit line 120 b, a channel region 123 comprising doped semiconductor material on the first terminal 122, and a second terminal 124 comprising doped semiconductor material on the channel region 123.

A conductive cap 127 comprising silicide is on the second terminal 124. The conductive cap 127 may comprise, for example, a silicide containing Ti, W, Co, Ni, or Ta. The conductive cap 127 provides a low resistance contact between the doped semiconductor material 126 and an electrode 250.

In the illustrated embodiment the first and second terminals 122, 124 comprise highly doped N-type material, and the channel region 123 comprises doped P-type material.

The first and second terminals 122, 124, the channel region 123, and the conductive cap 127 form a stack which is surrounded by a dielectric 230, the dielectric 230 separating the channel region 123 from the corresponding word line 130 b.

The word lines 130, include word line 130 b acting as the gate of the field effect transistor 115 of the memory cell 110, extend into and out of the cross section illustrated in FIG. 2B and comprise doped polysilicon material and a silicide layer on the doped polysilicon. The stack formed by the first and second terminals 122, 124, the channel region 123, and the conductive cap 127 extends through a via in the word line 130 b to electrically couple the bit line 120 b to the electrode 250, the via in the word line 130 b having a sidewall surface 135 surrounding the channel region 123.

The electrode 250 is on the conductive cap 127 and extends through dielectric 270 to a memory element 125 comprising a portion of the programmable resistance memory material 290 of memory plane 295. The programmable resistance memory material may comprise, for example, one or more elements from the group of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn, Cu, Pd, Pb, Ag, S, Si, O, P, As, N and Au.

The electrode 250 may comprise, for example, TiN or TaN. TiN may be preferred in embodiments in which memory material 290 comprises GST (discussed in more detail below) because it makes good contact with GST, it is a common material used in semiconductor manufacturing, and it provides a good diffusion barrier at the higher temperatures at which GST transitions, typically in the 600-700° C. range. Alternatively, the electrode 250 may comprise, for example, one or more elements from the group of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, N, O, and Ru.

The conductive material 140 of the memory plane 295 is on the programmable resistance memory material 290 and is coupled to a common voltage. In embodiments the conductive material 140 may comprise one or more conductive layers each comprising, for example, one or more elements from the group of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, N, O, and Ru. Advantages of having at least two conductive layers for the conductive material 140 include choosing the material of a first conductive layer for compatibility with the memory material 290 of the memory plane 295, while material of a second conductive layer on the first conductive layer can be chosen for other advantages such as higher electrical conductivity than the first conductive layer.

In operation, the common voltage coupled to the conductive material 140 and voltages supplied to the word line 130 b and the bit line 120 b can induce current to flow from the bit line 120 b to the conductive material 140, or vice versa, via the first terminal 122, channel region 123, second terminal 124, conductive cap 127, electrode 250, and memory material 290.

The active region 128 is the region of the memory element 125 in which the memory material is induced to change between at least two solid phases. As can be appreciated, the active region 128 can be made extremely small in the illustrated structure, thus reducing the magnitude of current needed to induce a phase change. The thickness 292 of the memory material 290 can be established using thin film deposition techniques. In some embodiments the thickness 292 is less than 100 nm, for example being between 10 and 100 nm. Furthermore, the electrode 250 has a width 252 less than that of the conductive cap 127, and preferably less than a minimum feature size for a process, typically a lithographic process, used to form the word lines 130 of the array 100. Thus, the electrode 250 has a top surface contacting the memory material 290 of the memory plane 295, the top surface of the electrode 250 having a surface area less than the top surface of the conductive cap 127. The small top surface of the electrode 250 concentrates current density in the portion of the memory plane 290 adjacent the electrode 250, thereby reducing the magnitude of the current needed to induce a phase change in the active region 128. Additionally, the dielectric 270 may provide some thermal isolation to the active region 128, which also helps to reduce the amount of current necessary to induce a phase change.

As can be seen in FIGS. 2A-2B, the active region 128 has a “mushroom” shape, and thus the configuration of the electrode 250 and the memory element 125 is commonly referred to as a mushroom-type configuration. Alternatively, other types of configurations can be used.

FIGS. 2C and 2D illustrate cross-sectional views an alternative embodiment in which the electrodes 250 of the array of FIGS. 2A and 2B are omitted and the memory material 290 of the memory element 125 extends within the opening in the dielectric 270 to contact the conductive cap 127, resulting in a pore-type cell.

In the cross-sectional views of FIGS. 2A-2B the programmable resistance memory material 290 is a blanket layer of programmable resistance memory material extending across to contacting the electrodes 250 of the memory cells of the array 100, and thus the array 100 is not subject to the etching damage problems discussed above. In FIGS. 2A-2B the conductive material 140 comprises a blanket layer of conductive material on the blanket layer of programmable resistance memory material. In some embodiments the memory material 290 and the conductive material 140 may be patterned, for example, to form patches, strips or grids, the formation of patches, strips or grids removing memory material that is spaced away from the active regions so that the active regions are not subject to etch damage.

The channel regions 123 have a top view cross-sectional channel area which in the illustrated embodiment is defined by a first dimension 224 along a first direction along the word lines 130 as shown in FIG. 2A, and a second dimension 226 along a second direction along the bit lines 120 perpendicular to the first direction as shown in FIG. 2B. In some embodiments the memory material 290 may be patterned into a plurality of memory patches each having a top view cross-sectional patch area. This patch area may be, for example, greater than or equal to ten times the top view cross-sectional area of the channel regions 123 so that the memory patches are shared among neighboring memory cells and the active regions are not subject to etch damage.

In yet other embodiments the conductive material 140 may be patterned, for example into strips or a grid structure, while maintaining a blanket layer of memory material for the memory plane 290.

As can be seen in FIG. 2A, because of the vertical channel structure of the field effect transistors the memory cell density along the word lines 130 b is determined by the width of the bit lines 120 and the separation distance between adjacent bit lines 120. As can be seen in FIG. 2B, the memory cell density along the bit lines 120 b is determined by the width of the word lines 130 and the separation distance between adjacent word lines 130. Thus, the cross-sectional area of the memory cells of the array 100 is determined entirely by dimensions of the word lines 130 and bit lines 120, allowing for a high memory density of the array.

Additionally, since the channel region 123 and the first and second terminals 122, 126 are arranged vertically the field effect transistor can have a small cross-sectional area while also providing sufficient current to induce phase change. The length of the channel of the device is determined by the height of the channel region 123 and can made small, while the width of the channel of the device is dependent upon the circumference of the channel region 123 and can be made relatively large compared to the length. Thus, a relatively large width-to-length ratio can be achieved such that higher reset current can be obtained.

Embodiments of the programmable resistance material 290 of the memory plane include phase change based memory materials, including chalcogenide based materials and other materials. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VIA of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from group IVA of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te_(a)Ge_(b)Sb_(100−(a-b)). One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky U.S. Pat. No. 5,687,112, cols. 10-11.) Particular alloys evaluated by another researcher include Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇ (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v. 3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistance properties. Specific examples of memory materials that may be useful are given in Ovshinsky '112 at columns 11-13, which examples are hereby incorporated by reference.

Chalcogenides and other phase change materials are doped with impurities in some embodiments to modify conductivity, transition temperature, melting temperature, and other properties of memory elements using the doped chalcogenides. Representative impurities used for doping chalcogenides include nitrogen, silicon, oxygen, silicon dioxide, silicon nitride, copper, silver, gold, aluminum, aluminum oxide, tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide. See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent Application Publication No. U.S. 2005/0029502.

Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly.

Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge₂Sb₂Te₅.

Other programmable resistance memory materials may be used in other embodiments of the invention, including other materials that use different crystal phase changes to determine resistance, or other memory materials that use an electrical pulse to change the resistance state. Examples include materials for use in resistance random access memory (RRAM) such as metal-oxides including tungsten-oxide (WO_(x)), NiO, Nb₂O₅, CuO₂, Ta₂O₅, Al₂O₃, CoO, Fe₂O₃, HfO₂, TiO₂, SrTiO₃, SrZrO₃, (BaSr)TiO₃. Additional examples include materials for use in magnetoresistance random access memory (MRAM) such as spin-torque-transfer (STT) MRAM, for example at least one of CoFeB, Fe, Co, Ni, Gd, Dy, CoFe, NiFe, MnAs, MnBi, MnSb, CrO₂, MnOFe₂O₃, FeOFe₂O₅, NiOFe₂O₃, MgOFe₂, EuO, and Y₃Fe₅O₁₂. See, for example, US Publication No 2007/0176251 entitled “Magnetic Memory Device and Method of Fabricating the Same”, which is incorporated by reference herein. Additional examples include solid electrolyte materials used for programmable-metallization-cell (PMC) memory, or nano-ionic memory, such as silver-doped germanium sulfide electrolytes and copper-doped germanium sulfide electrolytes. See, for example, N. E. Gilbert et al., “A macro model of programmable metallization cell devices,” Solid-State Electronics 49 (2005) 1813-1819, which is incorporated by reference herein.

An exemplary method for forming chalcogenide material uses PVD-sputtering or magnetron-sputtering method with source gas(es) of Ar, N₂, and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The deposition is usually done at room temperature. A collimator with an aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, a DC bias of several tens of volts to several hundreds of volts is also used. On the other hand, the combination of DC bias and the collimater can be used simultaneously.

An exemplary method for forming chalcogenide material uses chemical vapor deposition CVD such as that disclosed in US Publication No 2006/0172067 entitled “Chemical Vapor Deposition of Chalcogenide Materials”, which is incorporated by reference herein.

A post-deposition annealing treatment in a vacuum or in an N₂ ambient is optionally performed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges from 100° C. to 400° C. with an anneal time of less than 30 minutes.

FIGS. 3-11 illustrate steps in a fabrication sequence suitable for manufacturing an array of memory cells illustrated in FIGS. 2A-2B.

FIGS. 3A and 3B illustrate cross-sectional and top views respectively of forming a substrate 200 comprising a well 205 having a first conductivity type and trench isolation structures 232 within the well 205 and extending into and out of the cross-section illustrated in FIG. 3A. The well 205 can be formed by implantation and activation annealing processes as known in the art. In the illustrated embodiment the well comprises doped P type material of silicon substrate 200.

Next, the bit lines 120 are formed within the well 205, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 4A and 4B. In the illustrated embodiment the bit lines 120 are formed by ion implantation to form doped regions having a conductivity type opposite that of the well 205.

Next, dielectric material 260 is formed on the structure illustrated in FIGS. 4A-4B, and word line material is deposited and patterned to form word lines 130, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 5A and 5B. In the illustrated embodiment the word line material comprises polysilicon and optionally a layer of silicide.

Next, dielectric 262 is formed on the structure illustrated in FIGS. 5A-5B and the dielectric 262 is planarized using, for example, Chemical Mechanical Polishing (CMP). An array of vias 600 are then formed through the word lines 130 to expose portions of the bit lines 120, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 6A and 6B. In FIGS. 6A and 6B the vias 600 have a circular-cross section. However, in embodiments the openings 600 may have a cross-section that is circular, elliptical, square, rectangular or somewhat irregularly shaped, depending on the manufacturing technique applied to form the openings 600.

Next, a layer 700 of dielectric material is formed on the structure illustrated in FIGS. 6A-6B including within the array of vias 600, and a sacrificial layer 710 comprising silicon is formed on the layer 700, resulting in the structure illustrated in the cross-sectional view of FIG. 7.

Next, the layer 700 and silicon 710 are anisotropically etched to form dielectric spacers 230 comprising material of layer 700 within the vias 600, and the remaining material of the second layer 710 is then selectively removed, resulting in structure illustrated in the cross-sectional and top views of FIGS. 8A and 8B. In the illustrated embodiment the second layer 710 comprises silicon and may be selectively removed by wet etching using, for example, KOH or THMA.

Next, an epitaxial process is performed on the structure illustrated in FIGS. 8A and 8B including within the vias 600, and a planarization process such as CMP is performed to expose a top surface of dielectric 262, thereby forming doped pillars within the vias 600 and on the top surfaces of the bit lines 120. The doped pillars have a first conductivity type. Dopants having a second conductivity type are then implanted within a portion of the doped pillars, and the remaining portions of the doped pillars having the first conductivity type are the doped regions 122. Dopants having the first conductivity type are then implanted within an upper portion of the doped pillars to form the doped regions 124, and the remaining portions of the doped pillars having the second conductivity type are the doped regions 123. A conductive cap 127 comprising silicide is formed on the doped region 124, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 9A and 9B. In the illustrated embodiment the doped regions 122 and 124 comprise N-doped silicon material, and the doped region 123 comprises P-doped silicon material. The conductive cap 127 comprises a silicide containing, for example, Ti, W, Co, Ni, or Ta. In one embodiment the conductive caps 127 comprise cobalt silicide (CoSi) and are formed by depositing cobalt and performing a rapid thermal process (RTP) such that the cobalt reacts with the silicon of the doped regions 124 to form the conductive caps 127. It is understood that other silicides may also be formed in this manner by depositing titanium, arsenic, doped nickel, or alloys thereof, in a manner similar to the example described herein using cobalt.

Next, a dielectric layer 270 is formed on the structure illustrated in FIGS. 9A-9B and openings 1000 having respective widths 1010 are formed to expose of a portion of the conductive caps 127, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 10A and 10B.

The openings 1000 having a sublithographic widths 1010 can be formed, for example, by forming an isolation layer on the dielectric 270 and forming a sacrificial layer on the isolation layer. Next, a mask having openings close to or equal to the minimum feature size of the process used to create the mask is formed on the sacrificial layer, the openings overlying the locations of the openings 1000. The isolation layer and the sacrificial layer are then selectively etched using the mask, thereby forming vias in the isolation and sacrificial layers and exposing a top surface of the dielectric 270. After removal of the mask, a selective undercutting etch is performed on the vias such that the isolation layer is etched while leaving the sacrificial layer and the dielectric 270 intact. A fill material is then formed in the vias, which due to the selective undercutting etch process results in a self-aligned void in the fill material being formed within each via. Next, an anisotropic etching process is performed on the fill material to open the voids, and etching continues until the dielectric 270 is exposed in the region below the vias, thereby forming a sidewall spacer comprising fill material within each via. The sidewall spacers have an opening dimension substantially determined by the dimensions of the voids, and thus can be less than the minimum feature size of a lithographic process. Next, the dielectric 270 is etched using the sidewall spacers as an etch mask, thereby forming openings 1000 having a widths 1010 less than the minimum lithographic feature size. The isolation layer and the sacrificial layer can be removed by a planarization process such as CMP, resulting in a structure as illustrated in FIGS. 10A-10B. See, for example, U.S. Pat. No. 7,351,648 and U.S. patent application Ser. No. 11/855,979, which are incorporated by reference herein.

Next, electrodes 250 are formed within the openings 1000 in the dielectric layer 270 to contact the conductive caps 127 of FIGS. 10A and 10B, resulting in the structure illustrated the cross-sectional and top views of FIGS. 11A and 11B. The electrodes 250 can be formed, for example, by depositing electrode material on the structure illustrated in FIGS. 10A and 10B using Chemical Vapor Deposition, followed by a planarizing process such as CMP. In embodiments in which the openings 1000 are formed using an isolation layer and a sacrificial layer as described above, in alternative embodiments the electrode material may be deposited within the openings 1000 and overlying the isolation layer and the sacrificial layer. A subsequent planarization process such as CMP can then remove the isolation layer and the sacrificial layer and result in the structure illustrated in FIGS. 11A-11B.

Next, memory material 290 can be formed on the structure illustrated in FIGS. 11A-11B by blanket depositing a layer of memory material, and the conductive material 140 can be formed by blanket depositing one or more layers of conductive material 140 overlying the memory material 290, resulting in the structure illustrated in FIGS. 2A-2B.

In an alternative embodiment, the step of forming the electrodes within the openings 1000 in the dielectric layer 270 of FIGS. 11A-1B is omitted. Instead, memory material 290 is formed on the structure illustrated in FIGS. 10A-10B including within the openings 1000, and conductive material 140 is formed on the memory material 290, resulting in the structure illustrated in FIGS. 2C-2D.

FIGS. 12-18 illustrate an alternative manufacturing embodiment to that of FIGS. 5-9 for forming the word lines 130 and the terminals and channel region of the transistors.

Referring to FIGS. 4A-4B, an epitaxial layer of silicon is formed on the structure illustrated in FIGS. 4A-4B and a passivating oxide 1250 is formed on the layer of silicon. Next, implantation and activation annealing processes are performed to form doped layers 1200, 1210, and 1220 within the layer of silicon, resulting in the structure illustrated in the cross-sectional view of FIG. 12. In the illustrated embodiment the layers 1200 and 1220 comprise doped N-type material, and layer 1210 comprises doped P-type material.

Next, the layers 1200, 1210, 1220, 1250 are patterned to form stacks 1300 of doped regions 122, 123, 126 on the bit lines 120, resulting in the structure illustrated in the cross-sectional view of FIG. 13.

Next, a layer of dielectric is formed on the stacks 1300 and the dielectric is anisotropically etched to form dielectric spacers 1400 on the sidewalls and surrounding the stacks 1300, resulting in the structure illustrated in the cross-sectional view of FIG. 14.

Next, a conductive layer 1500 comprising silicide is formed on the regions of the bit lines 120 between the dielectric spacers 1400, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 15A-15B. The silicide of the conductive layer 1500 helps to increase the electrical conductivity of the bit lines 120 and thus reduces the loading of the bit lines 120 and improves the uniformity of the array. The top conductive layer 1500 comprises a silicide containing, for example, Ti, W, Co, Ni, or Ta. In one embodiment the top conductive layer 1500 comprises cobalt silicide (CoSi) and is formed by depositing cobalt and performing a rapid thermal process (RTP) such that the cobalt reacts with the silicon of the doped regions of the bit lines 120. It is understood that other silicides may also be formed in this manner by depositing titanium, arsenic, doped nickel, or alloys thereof, in a manner similar to the example described herein using cobalt.

Next, dielectric 1600 is formed on the structure illustrated in FIGS. 15A-15B and the dielectric 1600 is etched back to expose a portion of the dielectric spacers 1400, resulting in the structure illustrated in the cross-sectional view of FIG. 16.

Next, word line material 1700 comprising polysilicon is deposited on the structure illustrated in FIG. 16, followed by a planarizing process such as CMP to expose a top surface of the doped regions 126, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 17A-17B.

Next, the word line material 1400 is patterned and a silicide process is performed to form conductive caps 127 on the doped regions and a conductive layer on the polysilicon of the word lines 130, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 18A-18B.

FIGS. 19A and 19B illustrate cross-sectional views of a portion of another embodiment of memory cells (including representative memory cell 110 arranged in the array 100, FIG. 19A taken along the word lines 130 and FIG. 19B taken along the bit lines 120.

In FIGS. 19A-19B the array 100 includes a memory region 1900 and periphery region 1910 on the single-crystalline semiconductor substrate 200. The substrate 200 has a substantially planar top surface 201. As used herein, the term “substantially planar” is intended to accommodate manufacturing tolerances during the formation of the substrate 200. The term “substantially planar” is also intended to accommodate manufacturing processes performed following the formation of the substrate 200 which may cause variations in the planarity of the top surface 201.

The periphery region 1910 includes logic device 1986 having a gate structure 1987 on a gate dielectric layer 1993. The gate dielectric layer 1993 is on the top surface 201 of the substrate 200. The gate structure 1987 comprises a layer of doped polysilicon on the gate dielectric layer 1993, and a layer of silicide on the doped polysilicon.

The logic device 1986 includes doped regions 1988, 1989 within the substrate 200 acting as the source and drain regions. A dielectric 1996 comprising one or more layers of dielectric material is on the logic device 1986.

Contact 1965 is coupled to doped region 1989 and extends to the top surface of dielectric 1996 to line 1960. Contact 1995 is coupled to the doped region 1988 and extends to the top surface of dielectric 1996 to line 1997. The line 1997 extends into memory region 1900 and is coupled to the conductive material 140 of the memory plane 295 by contact 1950 extending through dielectric 1996.

As can be seen in FIGS. 19A and 19B, the first terminals 122 of the access transistors in the memory region 1910 and the gate dielectric layer 1993 are both on the substantially planar top surface 201 of the substrate. As described in more detail below with reference to FIGS. 20 to 30, the logic device 1986 in the periphery region and the memory cells having vertical channels can be manufactured concurrently. As a result, the memory device has a reduced complexity and addresses design integration issues of periphery and memory regions, thereby reducing the cost.

In FIGS. 19A-19B the memory cells are implemented in a mushroom-type configuration. Alternatively, other types of configurations can be used. In one alternative embodiment the electrodes 250 of the array of FIGS. 19A-19B are omitted and the memory material 290 of the memory elements extends within the opening in the dielectric 270 to contact the conductive cap 127, resulting in a pore-type cell like that shown in FIGS. 2C-2D.

FIGS. 20 to 28 illustrate steps in a fabrication sequence suitable for manufacturing an array of memory cells illustrated in FIGS. 19A-19B.

FIG. 20 illustrates a step including forming a substrate 200 comprising a well 205 and trench isolation structures 232 within the well 205 and extending into and out of the cross-section illustrated in FIG. 20. The well 205 can be formed by implantation and activation annealing processes as known in the art. In the illustrated embodiment the well comprises doped P type material of silicon substrate 200. The substrate 200 has top surface 201.

Next, gate dielectric layer 1993 is formed on the top surface 201 of the periphery region 1910 of the substrate 200 of FIG. 20. Gate structure 1987 is formed by depositing and patterning doped polysilicon material, and then forming a conductive cap comprising silicide on the doped polysilicon material, resulting in the structure illustrated in the cross-sectional view of FIG. 21. Alternatively, other techniques may be used to form the gate structure 1987.

Next, the bit lines 120 are formed within the well 205 and doped regions 1988, 1989 acting as the source and drain are formed within the periphery region 1910, resulting in the structure illustrated in the cross-sectional view of FIG. 22. In the illustrated embodiment the bit lines 120 and doped regions 1988, 1989 are formed by ion implantation.

Next, dielectric 2300 is formed on the structure illustrated in FIG. 22 and a plurality of openings 2310 are formed in the dielectric 2300 to expose portions of the bit lines 120, resulting in the structure illustrated in FIG. 23. The dielectric 2300 may comprise, for example, boro-phospho-silicate glass (BPSG) or PSG.

Next, a selective epitaxial process is performed within the openings 2310 to form doped regions (first terminals) 122 on the bit lines 120, resulting in the structure illustrated in the cross-sectional view of FIG. 24. In the illustrated embodiment the doped regions 122 comprise N-type doped silicon.

Next, another selective epitaxial process is performed within the openings and a planarizing process such as CMP is performed to form doped pillars 2500, resulting in the structure illustrated in the cross-sectional view of FIG. 25. The doped pillars 2500 have a conductivity type opposite that of the doped regions 122, and in the illustrated embodiment comprise P-type doped silicon.

Next, an implantation process is performed to implant dopants within an upper portion of the pillars 2500 to form doped regions (second terminals) 124 having the same conductivity type as the doped regions 122, resulting in the structure illustrated in the cross-sectional view of FIG. 26. The remaining portions of the pillars 2500 between the doped regions 122 and 124 are the channel regions 123 of the access transistors.

Alternatively, the doped regions 122, 124 and channel regions 123 may be formed using a single selective epitaxial process, rather the two selective epitaxial processes used in the illustrated embodiment of FIGS. 24-26. For example, in one such alternative embodiment a selective epitaxial process is performed within the openings 2310 of the structure of FIG. 23 to form doped pillars filling the openings 2310, the doped pillars and having a first conductivity type. Next, dopants are implanted within the doped pillars to form the channel regions having a second conductivity type opposite the first conductivity type, and form the second terminals on the channel regions and having the first conductivity type. The portions of the doped pillars underlying the channel regions are the first terminals.

Referring back to the structure illustrated in FIG. 26, next a portion of dielectric 2300 is removed to expose outer surfaces of the doped regions 122, 123, 124 and dielectric 2700 is grown on the exposed outer surfaces, resulting in the structure illustrated in the cross-sectional view of FIG. 27.

Word line material, for example polysilicon, is then deposited on the structure illustrated in FIG. 27, and planarized to expose a top surface of doped regions 124. The word line material is then patterned and a silicide process is performed to form conductive caps 127 on the doped regions 124 and conductive layers on the tops of the word lines, resulting in the structure illustrated in the cross-sectional and top views of FIGS. 28A and 28B.

Next, dielectric material 270, electrodes 250, and memory plane 295 are formed, for example, as discussed above with reference to FIGS. 10A-11B. Dielectric 1996 is then formed, contacts 1950, 1995, 1965 are formed, and conductive lines 1997 and 1960 are formed, resulting in the structure illustrated in the cross-sectional views of FIGS. 19A-19B.

Since the logic devices in the periphery region and the memory cells having vertical channel access transistors in the memory region are manufactured concurrently in the manufacturing steps described, the memory device has a reduced complexity and addresses design integration issues of periphery and memory regions.

FIG. 29 is a simplified block diagram of an integrated circuit 2910 including a memory array 2912 implemented using memory cells having a memory plane overlying vertical channel field effect transistor access devices as described herein. A memory plane termination circuit 2970 is coupled to the array and provides a common voltage to the memory plane of the array 2912. A word line decoder 2914 having read, set and reset modes is coupled to and in electrical communication with a plurality of word lines 2916 arranged along rows in the memory array 2912. A bit line (column) decoder 2918 is in electrical communication with a plurality of bit lines 2920 arranged along columns in the array 2912 for reading, setting, and resetting the phase change memory cells (not shown) in array 2912. Addresses are supplied on bus 2922 to word line decoder and drivers 2914 and bit line decoder 2918. Sense amplifiers and data-in structures in block 2924, including voltage and/or current sources for the read, set, and reset modes are coupled to bit line decoder 2918 via data bus 2926. Data is supplied via a data-in line 2928 from input/output ports on integrated circuit 2910, or from other data sources internal or external to integrated circuit 2910, to data-in structures in block 2924. Other circuitry 2930 may be included on integrated circuit 2910, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by array 2912. Data is supplied via a data-out line 2932 from the sense amplifiers in block 2924 to input/output ports on integrated circuit 2910, or to other data destinations internal or external to integrated circuit 2910.

A controller 2934 implemented in this example, using a bias arrangement state machine, controls the application of bias arrangement supply voltages and current sources 2936, such as read, program, erase, erase verify and program verify voltages and/or currents. Controller 2934 may be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, controller 2934 comprises a general-purpose processor, which may be implemented on the same integrated circuit to execute a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized for implementation of controller 2934.

While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. 

1. A memory device comprising: a plurality of bit lines; a plurality of word lines overlying the plurality of bit lines; a plurality of field effect transistors, field effect transistors in the plurality of field effect transistors comprising: a first terminal electrically coupled to a corresponding bit line in the plurality of bit lines; a second terminal overlying the first terminal; a channel region separating the first and second terminals and adjacent a corresponding word line in the plurality of word lines, the corresponding word line acting as a gate of the field effect transistor, and wherein the channel region has a top view cross-sectional channel area; a dielectric separating the corresponding word line from the channel region; and a memory plane comprising programmable resistance memory material electrically coupled to respective second terminals of the field effect transistors, and conductive material on the programmable resistance memory material and coupled to a common voltage, wherein the memory plane comprises a plurality of memory patches of programmable resistance memory material, memory patches in the plurality of memory patches having a top view cross-sectional patch area greater than or equal to ten times the top view cross-sectional channel area.
 2. The device of claim 1, further comprising a plurality of vias within the word lines, respective vias having a sidewall surface surrounding the channel region of the corresponding field effect transistor.
 3. The device of claim 2, wherein the first and second terminals and channel region of the field effect transistors form a stack extending through the corresponding via.
 4. The device of claim 1, further comprising a plurality of electrodes electrically coupled to the respective second terminals of the field effect transistors.
 5. The device of claim 4, further comprising a conductive cap comprising silicide on the second terminals of the field effect transistors, the plurality of electrodes extending through a second dielectric to contact top surfaces of respective conductive caps of the field effect transistors.
 6. The device of claim 5, wherein the plurality of electrodes have respective top surfaces contacting the programmable resistance memory material of the memory plane, the top surfaces of the electrodes having a surface area less than that of top surfaces of the corresponding conductive caps.
 7. The device of claim 1, wherein the programmable resistance memory material extends through openings in a second dielectric overlying the respective second terminals of the field effect transistors.
 8. The device of claim 1, wherein the memory plane comprises a blanket layer of the programmable resistance memory material overlying the respective second terminals of the field effect transistors.
 9. A method for manufacturing a memory device, the method comprising: forming a plurality of bit lines; forming a plurality of word lines overlying the plurality of bit lines; forming a plurality of field effect transistors, forming field effect transistors in the plurality of field effect transistors comprising: forming a first terminal electrically coupled to a corresponding bit line in the plurality of bit lines; forming a second terminal overlying the first terminal and acting as a drain or source; forming a channel region separating the first and second terminals and adjacent a corresponding word line in the plurality of word lines, the corresponding word line acting as a gate of the field effect transistor, and wherein the channel region has a top view cross-sectional channel area; forming a dielectric separating the corresponding word line from the channel region; and forming a memory plane comprising forming programmable resistance memory material electrically coupled to respective second terminals of the field effect transistors, and forming conductive material on the programmable resistance memory material and coupled to a common voltage, and wherein the memory plane comprises a plurality of memory patches of programmable resistance memory material, memory patches in the plurality of memory patches having a top view cross-sectional patch area greater than or equal to ten times the top view cross-sectional channel area.
 10. The method of claim 9, further comprising forming a plurality of vias within the word lines, respective vias having a sidewall surface surrounding a channel region of a corresponding field effect transistor.
 11. The method of claim 10, wherein the first and second terminals and channel region of the field effect transistors form a stack extending through the corresponding via.
 12. The method of claim 9, further comprising forming a plurality of electrodes electrically coupled to the respective second terminals of the field effect transistors, wherein forming the programmable resistance memory material comprises forming the programmable resistance memory material contacting top surfaces of the electrodes.
 13. The method of claim 12, further comprising forming a conductive cap comprising silicide on the second terminals of the field effect transistors, wherein forming the plurality of electrodes comprises forming the plurality of electrodes extending through a second dielectric to contact top surfaces of respective conductive caps of the field effect transistors.
 14. The method of claim 13, wherein the plurality of electrodes have respective top surfaces contacting the programmable resistance memory material of the memory plane, the top surfaces of the electrodes having a surface area less than that of top surfaces of the corresponding conductive caps.
 15. The method of claim 9, wherein forming the programmable resistance memory material comprises forming the programmable resistance memory material within openings in a second dielectric overlying the respective second terminals of the field effect transistors.
 16. The method of claim 9, wherein forming the memory plane comprises: forming a blanket layer of the programmable resistance memory material; and forming a blanket layer of the conductive material on the blanket layer of the programmable resistance memory material.
 17. The method of claim 9, wherein the forming the plurality of word lines and the forming the plurality of field effect transistors comprise: forming a second dielectric on the plurality of bit lines; forming the plurality of word lines on the second dielectric; forming a plurality of openings in the word lines to expose portions of a top surface of the plurality of bit lines; forming the dielectric on sidewalls of the openings; and forming the first and second terminals and the channel regions of the respective field effect transistors within corresponding openings in the plurality of openings, the first terminals contacting the top surface of the plurality of bit lines.
 18. The method of claim 17, wherein the forming the dielectric along sidewalls of the openings comprises: forming the dielectric on the sidewalls of the openings and on the exposed portions of the top surface of the plurality of bit lines; and forming sacrificial material on the layer of the dielectric; anisotropically etching the dielectric and the sacrificial material to expose portions of the top surface of the plurality of bit lines; and removing remaining portions of the sacrificial material.
 19. The method of claim 17, wherein the forming the first and second terminals and the channel regions of the respective field effect transistors within corresponding openings in the plurality of openings comprises: performing an epitaxial process within the plurality of openings to form doped pillars on the top surfaces of the plurality of bit lines, the doped pillars having a first conductivity type; implanting dopants having a second conductivity type within a portion of the doped pillars, wherein remaining portions of the doped pillars having the first conductivity type are the first terminals; and implanting dopants having the first conductivity type within an upper portion of the doped pillars to form the second terminals, wherein remaining portions of the doped pillars having the second conductivity type are the channel regions.
 20. The method of claim 9, wherein the forming the plurality of word lines and the forming the plurality of field effect transistors comprise: forming first terminal material on the plurality of bit lines, channel region material on the first terminal material, and second terminal material on the channel region material; patterning the first and second terminal materials and the channel region material to form a plurality of stacks on the plurality of bit lines; forming the dielectric on outer surfaces of the stacks; depositing word line material surrounding the dielectric; and patterning the word line material to form the plurality of word lines. 